Half wave annular transducer



y 1956 1.. w. CAMP 2,746,026

HALF WAVE ANNULAR TRANSDUCER Filed Aug. 14, 1953 INVENTOR. Leo/7 W. CampATTORNEY nited States atent i HALF WAVE ANNULAR TRANSDUCER Leon WaltonCamp, Glendale, Calif., assignor to Bendix Aviation Corporation, NorthHollywood, Calif., a corporation of Delaware Application August 14,1953, Serial No. 374,211

'6 Claims. (c1. 340-8 This invention relates to transducers of theradially vibratile ring type in which the inner surface of the ring isexposed to a liquid medium by which waves are transmitted or receivedthrough one end of the ring. Such transducers are particularly useful inunderwater sound transmission and reception at ultrasonic frequencies,although their use is not limited to any particular frequency or fluidmedium.

An object of the invention is to increase the efficiency and reduce thecost and weight of transducers of the general type described.

Other more specific objects and features of the invention will appearfrom the description to follow.

As heretofore constructed, transducers of the type referred to have beenclosed at the rear end by a massive backing member, and the axial lengthof the ring has been made one-quarter wave length of sound in the liquidmedium contained within the ring at the resonant frequency of thelatter. Such transducers are eflicient and practicable but are heavierand more expensive than they would be if the massive backing membercould be eliminated. Heretofore, the backing member has been considereda necessary adjunct to obtain good acoustic efiiciency, and itsthickness, for best efficiency, should be one-quarter the wave length ofsound in it at the operat-' ing frequency. The wave length decreaseswith an increase in the density of the material, but even with a densemetal such as lead the backing plate should have a thickness of 0.125in. for a transducer operating at 50 kc. Such a plate constitutes asubstantial portion of the total weight of the transducer and addsmaterially to its cost. At lower frequencies it becomes increasinglyheavy.

In accordance with the present invention, the massive backing plate canbe eliminated and the efficiency actually improved by making thevibratile ring one-half wave length long, and substituting for themassive backing plate a body of substantially less density than theliquid medium within the ring. Where the liquid medium is water or othersubstance of similar acoustic transmission properties, the backing bodymay be a gas or material having essentially the acoustic properties ofgas, such as air cell rubber or the equivalent. The thickness of thebacking member is preferably approximately 0.125 inch, but it is notcritical.

The new construction has proved to have higher acoustic efliciency andbetter directional characteristics than equivalent quarter wavetransducers employing massive backing plates. Although the length of thevibratile rings is doubled, that is not a serious objection in mostunderwater transducers, either with respect to cost or bulk, and it ismore than compensated for by the elimination of the weight and cost ofthe backing plate.

Both the present invention and the prior art devices depend uponreflection of sound at the rear end of the ring or cylinder to augmentthe sound energy at the'front end and to prevent radiation or receptionat the-rear end. Sound reflection is produced by a change in acousticimpedance and is proportional to the magnitude of the change, and thereflected wave differs in phase depending upon whether the reflectionresults from an increase or a decrease in impedance. Whereas the priorart devices produced the reflection by providing a body of higherimpedance at the end of a quarter wave length cylinder or ring, thepresent invention produces it by a body of lower acoustic impedance atthe end of a half wave length cylinder orring.

A complete understanding ofthe invention may be had from the followingdetailed description, read in connection with the drawing in which:

Fig. 1 is a longitudinal section through a simple singleelementtransducer in accordance with the invention;

Fig. 2 is a cross section taken in the plane II-II of Fig. 3, showing aseven-element transducer;

Fig. 3 is a section taken along the line III-III of Fig. 2;

Fig. 4 is a view similar to Fig. 3 but showing a modified construction;and

Fig. 5 is a polar diagram showing the directional characteristics of atransducer in accordance with the present invention and a correspondingprior art (quarter wave) transducer.

Referring to Fig. 1, the invention in its simplest form may comprise aradially vibratile ring member 10 open at one end and exposed at itsinner surface through the open end to the liquid medium through whichsound is to be transmitted or received. This ring 10 may be a bariumtitanate ceramic having electrodes 11 and 12 on its inner and outersurfaces, respectively, connected by leads 13 and 18 to a suitabletransmitting or receiving circuit. Such a ring expands and contractscircumferentially when alternating current is applied to its electrodes11 and 12, and if the frequency of the current corresponds to themechanical resonant frequency of the ring, it is capable of vibrating atrelatively large amplitudes. The vibratile ring member may be of themagnetostrictive type if desired, since the present invention relates tothe dimensions and acoustic setting of the ring rather than to itsspecific electromechanical nature.

The outer surface of the ring 10 to which the electrode 12 is attachedis acoustically shielded by a layer of material 14 the liquid within thering, so that the major portion of the sound emanating from the outersurface is reflected. Likewise, the rear end of the ring is closed by alayer or wall 15 of a material having a low acoustical impedance. Boththe member 14 and wall 15 may, in practice, consist of air cell rubberor the equivalent. To provide mechanical support for the assembly andfurther shield the outer and rear surfaces acoustically from the wateror other medium in which the unit is immersed, a cup-shaped case 16 ofsome acoustic insulating material, such as corp-rene, may be provided.

The essential feature of the present invention resides in making thering or cylinder 11 of axial length equal to one-half wave length ofsound in the liquid medium within the ring at the operating frequencyand closing the rear end of the ring with the wall 15 of low acousticimpedance. It is found that this produces a higher efficiency at lowercost and with less weight than the prior art transducers in which thelength of the ring 10 was onequarter wave length and a massive wall,usually of metal, was employed in place of the wall 15.

Although the invention may be employed in a single unit device as shownin Fig. 1, it is usually preferable to employ an array of individualunits in order to increase the power capacity and improve thedirectional pattern. A transducer containing such an array of units isshown in Figs. 2 and 3. Here each unit 10 corresponds exactly to theunit 10 of Fig. 1, there being seven of these units positioned as shownin Fig. 2. Each ring 10 is of low acoustic impedance as compared to thatofsurrounded with a ring 14 of low impedance acoustic material, the sameas in Fig. 1, but the seven rings and their associated insulating rings14 are supported with respect; to each other in, a single member 17.- ofacoustic rubber or the like having recesses 17a in its rear face forreceiving the rings 10 and their surrounding rings 14 and retaining themin position with respect to each other. The. member 17 also. definesthin front wall sections 17b overlying each ring 10 and constituting asound Window through which sound is transmitted from a liquid, such as asuitable oil, contained within the rings 10, and the water or externalmedium through which sound: is. to be transmitted or received. The body17 may be supported by an annular frame 19' of metal or other relativelyrigid material which, in turn, is supported by a back wall member 20.The rear ends of the rings 1!). are all. closed by a single layer orwall 21 of material, such as air cell rubber, having a low acousticimpedance and which performs the function of the layer 15 in Fig. 1.This layer 21, which is usually of low mechanical strength, ismechanically supported against the body 17 and the rear ends of therings 10 by a relatively thin wall 22 of a some material havingsubstantial mechanical strength, such as steel. The leads 13 and 18 fromthe units 10 may be brought through the walls 21 and 22 into a cavity 23defined between the walls and 22, where they may be connected togetherand extended through a suitable cable 24 attached to the rear wall 20.

The modified construction shown in Fig. 4 is identical in all respectsto that shown in Fig. 3', except that two 9 layers 25 and 26 ofdiiferent materials are substituted for the single body 17 in Fig. 3.The body 25 is of thickness equal to the axial length of the vibratilerings 10 and encloses them. This body 25 may be of some relatively rigidacoustic material, suchas corprene. The wall 26 functions as a soundwindow to acoustically couple the liquid within the transducer to theexternal liquid and should be of. a rubber or. similar material havingsubstantially the same sound transmission properties as water.

In both Fig. 3- and Fig. 4-, all free space within the transducer may befilled with the same oil or other liquid which constitutes the liquidmedium filling the rings 10.

The following table shows the relative acoustic charaeter-istics of atransducer of the type shown in Fig. 3 and a prior art transduceridentical therewith, except that the rings 10 are quarter wave lengthand the backingmember 21" is of a dense material such as lead or steel:

t Half Wave Cavity Cavity Where:

where in and f are the half power frequencies above and below theresonant frequency.

The table shows that as compared to the quarter wave prior arttransducers, the new half-wave transducer has approximately20%.improvement in efficiency, a somewhat broader band width, and aslightly lower electrical Q. All of these features are definitelyadvantageous in the operation of an underwater transducer.

The improvement in directional, characteristics of the presenttransducer, as shown in Fig. 3, relative to the characteristics of acorresponding prior art quarter wave transducer is shown in. the polardiagram of Fig- 5, in which the solid line shows the characteristic ofthe present half wave transducer and the dotted line shows thecharacteristic of the corresponding prior art quarter wave transducer.It will be observed that the present transducer approaches the action ofa vibrating piston more closely than does the quarter wave transducer,and produces a superior pattern with lower secondary lobes.

Although for the purpose of explaining the invention, a particularembodiment thereof has been shown and described, obvious modificationswill occur to a person skilled in the art, and I do not desire to belimited to the exact details shown and described.

I c. aim: V

1. In a transducer for translating sound waves in a liquid body intoelectrical Waves in an electric circuit and vice versa: a hollow,generally cylindrical, open-ended radially vibratile member ofelectromechanically responsive material and means electricallyresponsive to radial vibration of said member for electrically couplingit to said electric circuit; means, including a sound-transmitting,effectively liquid medium within said member in contact with the innersurface thereof, defining a sound transmission path from said innersurface to said liquid body through one end of said member; and means ofsubstantially less acoustic impedance than said liquid medium closingthe other end of said member, said member being of axial lengthsubstantially equal to one-half wave length of sound in said liquid.medium at the natural frequency of radial vibration of said member.

2. A transducer according to claim 1 in which said.

means closing the other end of said member has substantially theacoustic impedance of a gas.

3. A transducer according to claim 1 in which said means closing theother end of said member comprises a wall of air cell rubber incontactwith said medium within said member.

4. A transducer according to claim 1 including an array of saidvibratile members in side-by-side relation; supporting means for saidmembers. comprising a first layer of acoustic insulating material havingcavities the walls of which are in supporting engagement with theperipheral outer surfacesv of said members; said means closing saidother ends of said members comprising a second continuous layer ofacoustic insulating material lying against the said other ends of saidmembers and the corresponding face of said first-mentioned layer.

5. Apparatus according to claim 4 including arelatively rigid thin walllying against the outer face of said second layer of. insulating.material and mechanically supporting it against said members and saidfirst layer.

6. Apparatus according to claim 5 including a front wall member ofsubstantially the same acoustic impedance as, said liquid medium lyingagainst said one. end of saidmembers and. the corresponding face of saidfirst-mentioned layer.

References Cited in the file of this patent UNITED STATES PATENTS2,076,330 Wood Apr. 6, 19 37 2,497,901 Mott Apr. 9, 1946'

